A resistance temperature device (RTD) is a device whose resistance varies according to its temperature. By measuring the resistance of a RTD at various temperatures, a Resistance vs. Temperature curve (RT curve) may be obtained for that RTD. Given the resistance of an RTD and its RT curve one can compute the temperature of that device.
One important value that may be obtained from an RT curve is a value known as alpha. Generally, alpha (sometimes referred to as the temperature coefficient of resistance (TCR)) represents the percent unit change in resistance per unit change in temperature (Ohms/Ohm/deg. C.). Graphically, the TCR at a given point represents the slope of the RT curve at that point.
While any device whose resistance varies with temperature may function as an RTD, there are several desirable features that improve an RTD's performance.
First, it is generally desired that the RTD's RT curve be substantially linear over the temperature range to be measured. As discussed above, the TCR corresponds to the slope of the RT curve. Thus, in order to have a substantially linear curve, the TCR of the RTD should be substantially constant over the desired temperature range.
This linearity is desired because, among other thing, it simplifies the development of electronic circuitry to convert the resistance of the RTD into an electrical signal which varies as a function of temperature and it enables the use of linear curve fitting techniques. Further, a substantially constant TCR is important for the precise measurement of temperature.
A second desirable feature of RTD's is that their RT curves be repeatable. This primarily means that the RT curve for a particular RTD be the same after every temperature cycle (or that the TCR of the RTD be substantially constant over time). For example, most RTD's expand or contract as the temperature changes. This expansion and contraction often causes the RT curve (i.e., the TCR) of a particular device to change over time. Thus, the RT curve of one specific RTD subject to several temperature fluctuation may be substantially different than it originally was. The RTD industry has established certain standards establishing the acceptable percent variability of the TCR for commercial RTDs. A change in the RT curve is undesirable because, among other things, the electrical circuitry used with the RTD is generally designed to operate with only one RT curve (i.e., one TCR).
Third, because of the wide range of applications in which RTD's are used, it is desirable that the RTD be durable and rugged. Temperature measurements must be made in many harsh environments, e.g., environments having harsh chemicals or strong vibrations, and it is desirable to have an RTD insensitive to such vibrations, chemicals and the like.
Fourth, it is generally desirable that the RTD respond rapidly to changes in temperature. In certain applications (e.g. fire detection, heat detection) it is important to detect the change in temperature almost immediately. Thus, a desired attribute of an RTD is rapid response to temperature changes.
Finally, it is generally desirable to have an RTD that can be easily and economically produced.
Prior art devices have been able to combine but a few of the above describe attributes in any one RTD. For example, the platinum wire RTD, developed by C. H. Myers in 1932, has a substantially constant TCR, is able to accurately measure temperature changes, but is fragile and often uneconomical. This device is produced by winding a helical coil of platinum wire on a crossed mica web, and mounting this assembly inside a glass tube. The winding of the wire on the web is necessary for a substantially constant and repeatable RT curve (i.e., the TCR is constant).
This particular arrangement, of an unsupported wire structure in a glass tube, is not very durable and is susceptible to strong vibrations. As such, the platinum wire RTD is not practicable for many applications. Further, the high cost of platinum renders the platinum wire RTD relatively expensive. Recent attempts have been made to increase the durability of platinum RTD's. According to the most recent construction techniques, a platinum or metal glass slurry is deposited onto a small flat ceramic substrate and sealed. One example of such a technique is provided by U.S. Pat. No. 4,139,833 to Kirsch. These ceramic based RTD's are more resistant to harsh environments than the platinum wire RTDs, but are often extremely brittle and rigid. Further, if the coefficient of thermal expansion of the ceramic substrate differs from that of the deposited metal or slurry, which it usually does, substantial changes in the RT curve may occur. These changes are a result of the stress and strain placed on the deposited material when the substrate and the material attempt to expand at different rates. In an attempt to avoid such a change in the RT curve, the substrate used in many prior art devices is selected such that it is substantially rigid (i.e., it does not expand or contract at operating temperatures).
In addition to ceramic based RTDs, prior art RTDs have used thin film devices as alternatives to the platinum wire RTD. U.S. Pat. No. 4,375,056 to Baxter et al. discloses a thin film RTD comprising a serpentine metal pattern deposited upon an insulating substrate. The deposited platinum is disclosed as being between the range of 0.05-0.8 microns, with depositions below 0.05 microns described as being too thin to be practically handled. As with the ceramic based RTDs discussed above, devices of the type disclosed in Baxter are susceptible to differences between the coefficient of thermal expansion of the deposition layer and the insulating substrate.
Each of the described prior art devices utilizes a thin (or thick) metal film deposited upon a rigid substrate. In order to prevent strain on the metal deposition, the substrate are generally selected such that they do not expand or contract at opening temperatures.
As discussed above, RTD's are based on the principle that certain materials exhibit a change in resistance for a change in temperature. In order to allow for accurate measurements of the resistance change, it is generally desirable for the RTD to exhibit a measurable degree of resistance. In the prior art metal is the material of choice for RTD's. Because most metals are fairly conductive, prior art devices generally follow one of two alternatives: first, prior art devices often utilize metals having a relatively high resistivity (e.g., platinum). Metals having low resistivity (i.e., metals that are highly conductive) such as gold, silver, and aluminum are rarely used.
Second, as opposed to using a metal with low conductivity, some prior art devices use a metal with moderate conductivity (e.g, copper). To compensate for the moderate resistance of copper, prior art device merely use more copper. Thus, while copper RTD's are often have TCRs similar to those of platinum, the length of the resistance element is substantially greater.
This focus on metals with low conductivity results in a dilemma for the prior art. The devices must either be made out of a metal having relatively low conductivity (e.g., platinum), resulting in high cost; or the devices must be made out of a metal having moderate conductivity (e.g., copper), requiring the use of a longer resistive element the often complex manufacturing processes associated with such lengthy elements. The use of highly conductive metals (e.g., gold, silver, aluminum) in RTDs was avoided in the prior art because of the size of the resistive elements that were believed to be necessary.
The prior art devices, while possessing many of the attributes of a desirable RTD, are not adapted for use in all possible applications. Specifically, the brittle nature of these prior art devices makes it difficult to conform the RTD to the shape of a particular object whose temperature is to be measured.
In contrast to the prior art, the present invention realizes all of the describes attributes desirable for a RTD. Further, the present invention provides an RTD that may be adapted to most all possible RTD applications.
It is a feature of the present invention to provide a RTD that has a substantially linear RT curve, has a substantially constant TCR, and is adaptable to operation in harsh environments. It is further feature of this invention to provide an RTD with a fast response time.
It is a still futher feature of this invention to provide an RTD which may be easily and economically produced using low cost, highly conductive metals.
Additional features of the present invention will be set forth in the description that follows, and may be learned by practice of the invention.